As the use of electronic communications, such as e-mail communications, has increased, so has the occurrence of unsolicited bulk communications, also called spam. While spam often is commercial in nature, it also includes non-commercial bulk communications, such as political or non-profit communications sent to large numbers of electronic communication users. To control and prevent spam, communication users employ spam filter systems. Two types of spam filter systems are black-listing systems and white-listing systems. A black-listing system maintains a list, called a black-list, of user names, e-mail addresses, domain names, or other identifying information for communication users such as these who are known or likely to send spam. Black-listing systems block communications from any user who appears on the black-list. Conversely, a white-listing system works by maintaining a list, called a white-list, of user names, e-mail addresses, domain names, or other identifying information for approved communication users. White-listing systems accept communications from any user who appears on the white-list.